As shown in FIG. 1, a direct current side of a five-level inverter 10 is connected to a photovoltaic array 20 and configured to receive a PV input voltage at the direct current side. The PV input voltage is applied across a positive electrode of a capacitor C1 and a negative electrode of a capacitor C2, voltages of C1 and C2 relative to a middle point of DC-bus are V1Pos and V1Neg respectively. And the PV input voltage is step up by two Boost circuits at the front of the five-level inverter 10, then is applied across a positive electrode of a bus capacitor C3 and a negative electrode of a bus capacitor C4, and voltages of C3 and C4 relative to the middle point of the DC-bus are V2Pos and V2Neg respectively. An output voltage from the grid side of the five-level inverter 10 is filtered by a filter 30, step up by a transformer 40 and then transmitted to a power grid. In different combinations of switching states of switching devices, the five-level inverter 10 alternately outputs levels +V1Pos, −V1Neg, +V2Pos, −V2Neg and a zero level corresponding to the middle point of the DC-bus.
In the conventional carrier based modulation method, based on the relationship between the voltage command value Vcmd of each phase bridge of the five-level inverter and the output voltage level, the modulation may include four modulation sections:
1) a first modulation section: in a case that Vcmd≥V1Pos, a switching device S2Pos and a switching device S1Pos are switched on alternately, and the output voltage level is switched between +V2Pos and +V1Pos;
2) a second modulation section: in a case that 0≤Vcmd<V1Pos, the switching device S1Pos and a switching device S0 are switched on alternately, and the output voltage level is switched between +V1Pos and 0;
3) a third modulation section: in a case that −V1Neg≤Vcmd<0, the switching device S0 and a switching device S1Neg are switched on alternately, and the output voltage level is switched between 0 and −V1Neg; and
4) a fourth modulation section: in a case that Vcmd<−V1Neg, the switching device S1Neg and a switching device S2Neg are switched on alternately, and the output voltage level is switched between −V1Neg and −V2Neg.
However, in a case that V1Pos−Vx<Vcmd<V1Pos, a corresponding duty ratio command value Dcmd meets 1−Vx/V1Pos<Dcmd<1; in a case that Vcmd is greater than 0 and approaches V1Pos from a value less than V1Pos, Dcmd is close to 1, thereby resulting in many narrow pulses; and in a case that Vx/V1Pos is less than the duty ratio Dthrs corresponding to the sum of narrow pulse time and dead-band time, in order to ensure that switching devices operate safely, these narrow pulses need to be deleted and Dcmd is clamped to 1. Similarly, in a case that Vcmd is greater than 0 and approaches +V1Pos from a value greater than +V1Pos, Vcmd is less than 0 and approaches −V1Neg from a value greater than −V1Neg, Vcmd is less than 0 and approaches −V1Neg from a value less than −V1Neg, or Vcmd approaches 0, there are also many narrow pulses to be deleted. However, deleting of narrow pulses results in distortion of the phase bridge output voltage.